Mound breakwaters are coastal structures built with concrete units and/or quarry stones of different sizes. The breakwaters are built in the sea or ocean in order to shelter a coastal area The basic elements of the mound breakwater are a core, filter layers atop the core, and armor, either with or without a cap on the crest of the breakwater.
Such a coastal structure causes the sea waves to break on the armor and, subsequently, to reflect and dissipate wave energy.
The core is the main part of the mound breakwater and comprises its largest volume part. The core is designed to resist the geotechnical conditions imposed by the sea bed, the concrete cap, the armor and the other layers of the breakwater. Additionally, the core should be impervious enough to prevent excessive wave transmission through the breakwater and it should also satisfy the hydrodynamic conditions of the sea waves inside the breakwater structure.
The filter layers comprise one layer or several layers of stones of decreasing sizes being of larger sizes outward toward the armor and of smaller size inward toward the core and sea bed. The filter layers are placed on the external surface of the core. Their function is to prevent the loss of smaller breakwater materials from the core through openings of the armor or large stone layers.
The armor, the external part of the breakwater, is placed atop the outer filter layer. The armor is a layer which protects the breakwater structure from the waves and is made of larger size units, such as larger stones. The armor is the basic resistant element of the mound breakwater It is characterized by the type of armor unit the shape, density, weight, gradation, resistance, etc. Stability also depends on the characteristics of the breakwater cross section.
A concrete cap, designed to reduce the waves overtopping the breakwater, may be placed on the breakwater crest.
Having herein defined a mound breakwater, it is important to point out the three basic types of mound breakwaters in existence:
1. The mound breakwaters of "classic design". PA0 2. The "S shaped" breakwaters also called mound breakwaters of the "broken profile" type. PA0 3. The "Berm type" breakwaters, also called "sacrificial type" breakwaters, or "reshaping" mound breakwaters.
Mound breakwaters of classic design have an armor of uniform slope from the bottom to the crest (which is with or without a cap). In these breakwaters of classic design, the armor thickness is approximately constant along the slope. The armor thickness is usually twice the equivalent cube size, or nominal diameter, although this may vary depending on the type of armor unit. Equivalent cube size, or nominal diameter, is the cube root of the average weight divided by the weight density of the armor units.
Mound breakwaters of classic design offer low permeability. One of their drawbacks is that if a sea wave storm exceeds initiation of the damage threshold level, and if the damage progresses into the armor, the permeability of the breaking zone is drastically reduced.
S shaped breakwaters have a nonuniform profile because the slope is flatter in the central part and steeper in the upper and lower parts. This profile looks like an "S" and is therefore termed an "S shaped" breakwater.
The S shaped breakwaters also have an armor of approximately uniform thickness. In comparison with conventional breakwaters, the special profile of S shaped breakwaters gives them a higher degree of resistance, although they have the disadvantage of needing a larger volume of materials for construction. The S shape does allow, however, for the reduction of the weight of the units of the armor, if one considers the same design wave storm for the initiation of damage, and a corresponding cost reduction. However, S shaped breakwaters are difficult to build because of the special required profile.
The third mound breakwater type, the "Berm type" breakwater, has a very large volume, although the stones of the armor may be much lighter than those of conventional rubble-mound breakwaters.
On the other hand, in the "Berm type" breakwaters, the action of the sea waves progressively forms the armor until stability is obtained. The names "reshaping" mound breakwater or dynamic breakwater refers to this process.
The principal advantages of the "Berm type" breakwaters are their reduced cost of construction and smaller required armor stones used. However, the "Berm type" breakwaters require a good deal more material to build, and their long term reliability is low due to longshore transport.